Hydrate inhibitors such as methanol and glycol are often injected into oil and gas high pressure production lines. For hydrate inhibitor distribution lines, rate control valves are often used to distribute fluid from one pump to several injection points to reduce the cost of pumps, piping systems, and pump maintenance. Flow rates for these valves will range from approximately 1/10th of a gallon per minute to up to eighty gallons per minute. In addition, wells can experience large pressure fluctuations during day-to-day operation.
Pressure compensated flow control valves are designed to maintain constant flow with changes in pressure drop across the device, wherein the flow passes to the underside of a throttling member, such as a mating cone and sharp edged seat (U.S. Pat. No. 6,662,823) and a sharp edged hollow cylinder (U.S. Pat. Nos. 4,250,915 and 5,642,752). In these flow control valves, the flow path is, as an example, over the throttle cone first and then through the mating seat, such that the valves are susceptible to inadequate control or inadvertent closure of the throttle upon a large pressure differential or a pressure spike in the fluid entering the valve. Accordingly, these pressure compensated valves are typically not designed to adequately handle large pressure drops across the valves.
U.S. Pat. Nos. 6,827,100 and 4,210,171 discloses control valves with fluid flow going under the seat first. These control valves, however, are not adequately balanced to handle large pressure drops across the valves or large, sudden pressure spikes (i.e., transient pressure spikes). As a result, the balance of these valves will become unstable with pressure spikes or large pressure drops across the valves.
Pressure balanced rate control valves, such as those disclosed in U.S. Pat. No. 4,893,649, Skoglund U.S. Pat. No. 5,234,025, and U.S. Pat. No. 6,932,107 are unique from other prior art pressure compensated rate controls because the ratio of the area balanced by the spring chamber is substantially larger than the area of the seat that dissipates the pressure drop. These pressure balanced rate control valves, however, have a configuration and flow direction such that the valves can go into a cyclic opening and closing sequence (sort of an on/off water hammer) with excessive pressure drops across the valve. This cyclic opening and closing can provide an undesirable harmonic cycling that will match the natural frequency of the piping supplying the valve.
A significant problem for conventional flow rate controllers is cavitation. Cavitation will typically occur in a valve trim if the fluid velocities are fast enough to cause the pressure at the velocity point to drop below the vapor pressure of the liquid. When pressure is dropped below vapor pressure it will create a collapsing bubble when the pressure is recovered. This collapsing bubble causes a point pressure load of up to 300,000 psi pressure on valve surfaces. This high contact pressure also causes an instantaneous heating at the collapsing bubble. The high heat and high contact pressure will erode the surfaces and will generate high frequency flow noises reverberating to the piping system. Cavitation and trim selection to avoid cavitation is described in ANSI/ISA-75.01-2002 “Flow Equations for Sizing Control Valves”. There is a need for a flow rate controller that effectively eliminates cavitation across its operating conditions.